Most modern outboard marine propulsion devices discharge exhaust gases and cooling water below the surface of the water. This design is particularly effective for preventing the obnoxious exhaust gases from travelling to the cockpit area of the vessel and disturbing passengers or crew members. Additionally, exhausting below the surface of the water serves to muffle the noise of the motor.
Many of these below-surface units discharge engine exhaust gases admixed with cooling water to the rear of the propeller area through the propeller hub. A principal benefit of this practice is that while the boat is in forward motion the fluid flow through the propeller is of uniform density (i.e., the water is not disrupted by bubbles of exhaust gas and the gases permit minimum negative pressure in the region of the hub). This advantage is achieved at some sacrifice to operation in the astern mode, wherein the exhaust gases are discharged upstream of the propeller and mix with the water flowing through the propeller area. The substantial decrease in density due to the bubbles reduces the reverse thrust capacity of the propulsion unit.
Several earlier attempts have been made to solve this problem of "cavitation" caused by the discharge of exhaust gases into the water through which the propeller is biting (i.e., upstream of the propeller). One such solution is presented in U.S. Pat. No. 3,467,051 which provides a marine propulsion lower unit having a means for axially shifting the propeller relative to the lower unit of the device. This axial shift of the propeller when the unit is in the reverse mode provides an opening downstream of the propeller through which exhaust gases discharge.
There are several disadvantages to the solution proposed in the patent. First, it is necessary to have a means for axially shifting the position of the propeller relative to the lower unit, thus requiring additional construction costs and efforts. Second, the proposed solution cannot readily be used to retrofit propulsion devices currently in use. Finally, the unit as designed could encounter serious fouling and plugging problems, particularly from vegetation which may become entangled around the propeller shaft which is exposed during reverse mode operation.